Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already cover safety (readOnly, idempotent, non-destructive), so the description's added value is the disclosed search strategy: caller-supplied selector first, then selectors observed in real mail, then 45 common ones. That tells the agent the call is a multi-step external probe that may take longer than a single lookup, which is genuinely useful context beyond the annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

A single sentence, front-loaded with the outcome, that packs the fallback order without filler. Nothing to cut.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

No output schema exists, so the description carries return-value burden, yet it never says what a hit or a miss looks like (key present? null? error for unregistered domain?). For a two-parameter read tool with annotations, the call mechanics are covered but the result shape is left to discovery.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 50% — domain is documented, selector is not. The description partly compensates by framing the selector as the first thing 'tries', implying it is an optional hint rather than a strict filter, but it doesn't give format, default, or what happens if the selector misses. Marginal but real added meaning.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

Specific verb (find) plus specific resource (DKIM key) for a domain, so an agent knows exactly what the tool retrieves. It does not explicitly differentiate itself from siblings like check_spf or check_dmarc, but the DKIM-specific wording is unmistakable.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description implies usage (domain DKIM verification) and explains the lookup strategy, but never states when to prefer this over check_dmarc/check_spf/domain_health, nor any prerequisites such as the domain being registered or DNS-managed here. Guidance is inferable rather than explicit.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.